In the construction of any battery, six elements must be present, namely, a positive and negative electrode, a housing, a separator, an electrolyte, and current collectors. The separator is an important element because it must be permeable to ions moving back and forth during charging and discharging, but must not permit flow of electrons directly from pole to pole, which shorts out the current, and prevents or limits electrons from flowing from anode to cathode through a circuit. In rechargeable lithium ion batteries, which are generally formed of very thin layers of the components, it is important that the distance between electrodes be as short as possible to permit efficient ion transfer, but not so short as to permit electron flow. Typically, separators have been constructed of very thin sheets of plastic, which are rendered porous.
In general, a nonaqueous environment is maintained, since lithium and its salts are notoriously reactive in aqueous solutions. Aprotic organic solvents such as propylene carbonate or ethylene carbonate are commonly used in which lithium salt solutes are readily dispersable. Other solvents are tetrahydrofuran, 1,2-dimethoxyethane, dimethyl carbonate, diethyl carbonate and diethoxyethane. For a discussion of conventional solvent/lithium solute systems, see S. Hossain, "Rechargeable Lithium Batteries (Ambient Temperature)", in Handbook of Batteries and Fuel Cells, D. Linden, Ed., McGraw-Hill, 2nd Ed., 1995. The plastic separator, of course, must be stable to the solvent selected.
U.S. Pat. Nos. 4,138,459, 3,801,404, and 3,843,761 disclose a method of producing a porous plastic separator by stretching a crystalline polyolefin film at a temperature below its transition temperature. Performance of such stretched polymers is impaired, however, because lack of control over uniform pore size leads to excessive distance between electrodes. U.S. Pat. No. 4,994,335 discloses a stretching process in which the strain rate and temperature of the process are tightly controlled, to produce microporosity in only one direction, so that fine fibrils are connected between adjacent unstretched planar flat portions in two dimensions.
A serious problem arises when the lithium ions react in the complete cell to form dendrites of lithium, which tend to short out the battery by filling the void spaces of the separator and creating a conductive pathway. U.S. Pat. No. 5,427,872 discloses a method of preventing dendrite shorting, by disposing an inert polypropylene or polyethylene composite separator along with a dendrite reactive fluorinated polymer such as polyfluoroethylene. Porosity is maintained by prevention of dendrite penetration of the electrode-protective second separator.
U.S. Pat. No. 5,290,644 discloses a separator made from a film composed of a cross-linked polymer such as polybutadione, etc. capable of swelling in the presence of electrolyte to create pressure against the electrodes. A woven or unwoven fabric which is capable of enhancing such swelling may optionally be incorporated into the film.
In an alternative approach, U.S. Pat. Nos. 5,460,904, 5,296,318, 5,429,891, 5,571,634, and 5,587,253 disclose a separator comprising a self-supporting film of a copolymer of vinylidene fluoride (PVdF) and hexafluoropropylene (HFP). Prior to casting, the copolymers are mixed with a medium to high temperature boiling plasticizer solvent such as dibutyl phthalate (DBP) and a filler such as SiO.sub.2. In a preferred embodiment, the "dry" separator may be joined with the electrodes by lamination under heat and pressure prior to electrolyte loading. After lamination, the plasticizer is leached out (extracted) with a solvent such as ethylether, and replaced by cell electrolyte. The spaces occupied by the plasticizer are filled with electrolyte in a communicating network of vacuoles permitting ion diffusion.
U.S. Pat. No. 4,550,064 discloses a separator comprising two layers, the first inner layer composed of microporous polypropylene (CELGARD.RTM.) or fiberglass whose surfaces are made more hydrophilic by coating with imidazoline. These separators are to be used in combination with a positive electrode manufactured with a propylene/ethylene elastomer binder. U.S. Pat. Nos. 4,650,730, 4,731,304, and 5,240,655 disclose separators made from bonding two extruded layers of film together. Extractable fillers are included in the extruded film so that upon extraction, the separator has the requisite porosity to function in a battery. U.S. Pat. No. 5,565,281 is an improvement in bilayer separators, in that one of the two layers is selected for its strength. The purpose of the bilayer is to provide shutdown capability in the event of a short, while maintaining the strength to resist breaches of contact between the electrodes.
The foregoing separators have certain disadvantages. Multi-layer separators have a step gradient of porosity with different diffusion constants for electrolyte in each layer. Efficiency of ion transfer may be impaired which interferes with the discharge rate of the battery, and may adversely affect capacity. In the case of the separators made from PVdF and HFP, solvent leaching causes brittleness which leads to a significant level of product failure.